1. Field of the Invention
This invention relates to focus detection devices for image forming optical systems, and more particularly to a device for detecting a sharp focus condition by comparing the outputs of at least two photo-sensitive elements on a common optical axis at respective axes of the conjugate point to the prescribed focal plane of the image forming optical system and its nearby points.
2. Description of the Prior Art
In prior art focus detection devices, the light splitter for routing part of the image bearing light bundle to the photo-sensitive elements is in the optical path of the image forming optical system, but behind the diaphragm. Such an arrangement, therefore, makes the accuracy of focus detection dependent upon the diaphragm aperture size of the image forming optical system. Particularly when the diaphragm stops down, focus detection accuracy is objectionably deteriorated.
Also, when applied to image forming optical systems of the zoom type, the light splitter system arrangement for the photo-sensitive elements in the optical path of the image forming optical system resulted in focus detection dependence on the focal length of the image forming optical system. In this situation, although accurate detection is attained in the telephoto positions, in the wide angle positions where the focus depth is deepened, poor detection results. In actual practice, after focus detection was performed in the wide angle positions, when zooming was subsequently carried out, the image was blurred as the object to be photographed was put outside the focus depth when in the telephoto positions.
A device for focus detection by providing a light path splitting means in part of the image forming optical system was disclosed in Japanese Laid-Open Patent Application Nos. Sho 57-54763, 57-192910 and 57-128312 and Japanese Laid-Open Utility Model Application Nos. Sho 55-57723 and 56-128622.
An object of the present invention is to provide a focus detection device which can always perform constant, accurate focus detection even when the diaphragm aperture size of the image forming optical system varies.
Another object of the present invention is to provide a focus detection device which can always perform accurate focus detection even when the focal length of the image forming optical system varies.